The controller area network (CAN) as well as an extension of the CAN, referred to as “time triggered CAN” (TTCAN), also collectively referred to below as the CAN standard, are known from the standards of the ISO 11898-1 through 11898-1 family, for example. The media access control process used in the CAN is based on bitwise arbitration. In the CAN, the bitwise arbitration is carried out based on a leading identifier within the message to be transmitted via the bus.
During the bitwise arbitration, multiple user stations may simultaneously transmit data via the bus system without interfering with the data transmission. At the same time, the user stations may ascertain the logic state (0 or 1) of the bus (reception signal) during transmission of a bit via the bus (transmission signal). For this purpose, the transmission signal sent on the transmission channel is continuously compared to the reception signal. If there is no match at a certain point in time (the sample point), the bus user terminates the transmission operation, since it must be assumed that another bus user is attempting to transmit a message having a higher priority or a lower-level identifier. The reception signal represents a superimposition of the message bits of all bus users which are attempting to gain access to the bus during the arbitration. Due to the propagation times of the signals on the bus lines and due to intrinsic delay times in the bus connection units (transceivers), the result of the superimposition of these signals is not present until late within the bit time period, so that the sample point must be relatively far back within the bit time period. Among other things, this fact limits the allowable bit lengths in the CAN at the lower end. Shortening is not easily possible.
After a user station has completely sent the identifier, it is established that this station has attained the arbitration and thus has exclusive access to the bus. According to the protocol specification of the CAN, other user stations are not allowed to send data via the bus until the sending user station has transmitted a check sum field (CRC field, CRC check sum) of the message. Thus, the end time of the transmission of the CRC field corresponds to an end of an interval in which the transmitting user station views as a reception signal only the transmission signal which that user station itself has sent, which is delayed essentially only by the intrinsic delay time of the bus connection unit. The described comparison between the transmission signal and the reception signal is further carried out in this interval, and is used to detect errors in the transmission of the data and to determine error messages by other bus users.
Use of the protocol results in an interference-free, secure transmission via the bus of that message whose sender has attained the arbitration process. The CAN protocols are particularly suited for transmitting brief messages under real-time conditions, whereby it may be ensured, by suitable assignment of the identifiers, that for particularly important messages arbitration is almost always attained, and the corresponding message is successfully sent.
For example, a high level of transmission security and reliability in error recognition is provided by transmitting a CRC field, formed from the data previously sent in the message, with the aid of a generator polynomial, and carrying out a CRC check on the receiver side, and by continuously checking the match between the transmission signal and the reception signal.
The increasing interlinking in recent vehicles and the advent of additional systems for enhancing driving safety or driving comfort, for example, have resulted in greater demands on the data volumes to be transmitted, the transmission rate, the transmission security, and the allowable latencies for the transmission. Examples include vehicle dynamics control systems such as the electronic stability program (ESP), driver assistance systems such as adaptive cruise control (ACC), or driver information systems such as traffic sign recognition (see, for example, the descriptions in “Bosch Kraftfahrtechnisches Handbuch (Automotive Handbook),” 27th Edition, 2011, Vieweg+Teubner).
The document “CAN with Flexible Data Rate, White Paper, Version 1.0” posted on the Internet site http://www.semiconductors.bosch.de/ on May 2, 2011 presents a modified data transmission protocol which, among other things, allows an expansion of the data field, as well as a shortening of the bit length for a portion of the CAN message after completed arbitration. In this area, the shortening of the bit length is limited, among other factors, by the intrinsic delay time of the bus connection units, since before a bit is transmitted, the match between the transmission signal and the reception signal must be checked for the preceding bit. It has been found that the related art does not provide satisfactory results in every respect with regard to increasing the data transmission rate and/or security.